X-ray laser uncovers secrets of complex oxide material

May 17, 2012 By Mike Ross

This diagram shows alternating stripes of charges and spins that self-organize in a particular nickel oxide at sufficiently low temperatures. This pattern constitutes a new quantum state, and it provides a model system that scientists can use to learn about electron correlations and their impact on the properties of materials.
Doped holes (dark red in the background) primarily reside on the nickel “3+” atoms (red circles) located in every fourth vertical row. This is called the “charge order” (CO).
The electron spins (arrows) of each of the next three rows of nickel “2+” atoms (gray circles) are oriented in opposite directions, that is, antiferromagnetically. The period of this “spin order” (SO) is twice that of the charge order. Credit: Wei-Sheng Lee

(Phys.org) -- An international team of researchers has used SLACs Linac Coherent Light Source (LCLS) to discover never-before-seen behavior by electrons in complex materials with extraordinary properties.

The result is an important step forward in the investigation of so-called strongly correlated materials, whose unusual qualities and futuristic applications stem from the collective behavior of their electrons. By understanding how these materials work, scientists hope to ultimately design novel materials that, for instance, conduct electricity with absolutely no resistance at room temperature, dramatically improving the performance and efficiency of energy transmission and electronic devices.

In a report published yesterday in Nature Communications, researchers led by SLAC Chief Scientist Zhi-Xun Shen and Lawrence Berkeley National Laboratory Scientist Zahid Hussain describe experiments at the LCLS with a material called striped nickelate.

It gets its name from the pattern of alternating stripes of enhanced charge and spin that its electrons collectively assume under certain conditions. This pattern constitutes a new quantum state, and it provides a model system that scientists can use to learn about electron correlations and their impact on the properties of materials.

The researchers hit the material with a pulse from an infrared laser, and then used an exceedingly intense, brief flash of X-ray laser light from LCLS  just a few millionths of a billionth of a second long  to record what happened.

The initial pulse jarred the nickelate out of its striped state. By varying the interval between the two pulses, the researchers created images that showed how the charge stripes reemerged. They were surprised to find that variations in the locations of minimum and maximum charge, controlled by a quantity called phase, persisted long after the stripes charge distribution returned to its original magnitude.

These phase fluctuations are very important for understanding how these materials behave, said Wei-Sheng Lee, a SLAC physicist and lead author on the research. But until now, they have been impossible to discern directly. Being able to see this electron behavior represents a new era in materials science research.

Other members of the research team come from the University of California-Berkeley, Lawrence Berkeley National Laboratory, Swiss Light Source, European X-ray Free-Electron Laser, Max-Planck Research Group for Structural Dynamics in Germany and the Tokyo Institute of Technology.

Yesterday's report is the fourth in the past six months describing experiments at LCLS that use pairs of optical and X-ray pulses to excite and probe materials that combine oxygen with so-called transition metals, such as nickel, copper, titantium or manganese.

While these transition-metal oxides can have many fantastic properties, the one that has caused the most scientific excitement is the prospect of being able to conduct electricity without resistance at much higher temperatures than is possible today.

Until 1986, all the known superconductors worked only at extremely low temperatures, limiting their usefulness. But that year, two Swiss scientists discovered that a copper-based oxide lost all its electrical resistance at 32 degrees Kelvin  about minus 400 degrees Fahrenheit. While this is still close to absolute zero, it was 12 degrees Kelvin higher than had ever been seen before.

The Swiss researchers received the Nobel Prize in Physics the very next year. In a frantic worldwide scramble, scientists found dozens of transition-metal oxide combinations that became superconducting at even higher temperatures  up to 135 degrees Kelvin at atmospheric pressure.

However, the dream of developing room-temperature superconductors for a new generation of magnetically levitated trains, superfast computers and super-efficient electrical power lines has not been realized, because no one knows why these complex materials behave as they do or how to predict their properties.

When scientists first looked at these materials 26 years ago, they had no clue that such spectacular properties would appear, Shen said. With LCLS, we now we have a new tool to help us learn how these properties arise.

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User comments : 7

This corresponds well with a prediction made by Art Winfree in the 1960's, though his prediction was not in the field of physics. Here, the period of the charge order is twice as fast as the period of the spin order. (As the article states, the spin period is twice as long as the charge period.) Winfree said that in a three oscillator system, there were four, and only four, patterns of self organization that would emerge. One was: two oscillators exactly antisynchronous to each other, and one oscillator at twice the frequency. In the picture above, look at the SO bar in the bottom. Take the first three dots in that five dot bar. The first three are gray up, red, gray down. That is the specific pattern that Winfree described. A good article on Winfree's work in Scientific American, December 1993, by Steve Strogatz and Ian Stewart. Relevant to many forms of self organization in physics, I suspect, including superconductivity.

because no one knows why these complex materials behave as they do or how to predict their properties

I know, how/why these material behave and many people know about it as well. For example the prof. E. Joe Eck, the founder of first room temperature superconductor, or Dr. J.F.Prins, the author of first artificial superconductor. Both these two people were expelled from community of physicists, so they're are staying outside of mainstream (if not against it) - but they're represent the top of experimental superconductor physics. It's symptomatic, their findings are ignored, their experiments aren't replicated in peer-reviewed-press (the similarity with cold fusion isn't accidental here at all) any you cannot read about it at mainstream press.

Winfree said that in a three oscillator system, there were four, and only four, patterns of self organization that would emerge

The question why superconductors do work can be explained easily with answering the question: why some elements can behave like the superconductors and some other not - even at the lowest temperatures and/or highest pressures? The Winfree's coupled oscillator hypothesis doesn't account to this question at all, because it takes account to one of the consequences of superconductivity - not its origin (after all, in the same way, like the famous phonon based BCS theory).

Well, the question really is as simple, as it is: why for example the sodium - an element full of electrons - can never form the superconductor (even if we squash it heavily) - whereas the brittle niobium element - a poorly conductive metal, which is rather similar to brittle ceramics than to "real metal" can form the superconductor at the relatively high temperatures easily?

The high level explanation of superconductivity is, the movable electrons must be compressed heavily in such a way, their repulsive forces overlap mutually. When the compression level is low, then the electrons tend to occupy the most effective position with respect to potential energy and they will form less or more regular lattice, where no electron can move freely - so called the Mott's insulator. But when the electrons are compressed heavily, then the quantum fluctuations of vacuum are able to overcome the residual energetic barriers, which do prohibit the electrons in their free motion and such a chaotic electron fluid will spread its charge in waves instead of individual particles like so-called the boson condensate.

But it's not easy to squeeze the electrons mutually, because these tiny particles can escape from even the tiniest holes of atom lattice. To prepare a superconductor, you should squeeze electrons within atom itself, i.e. with using of the cohesion of atom orbitals.

The secret of niobium atoms is, they do contain at least two types of orbitals: the compact spherical s- orbitals, and the elongates ones, which extent the diameter of the rest ones heavily. The elongated orbitals can couple mutually and they will form sorta cage, inside cells of which the electrons of s- orbitals can be squeezed mutually. This cage structure is responsible for the brittleness of metallic niobium too.

The sodium is metal full of electrons, its surface is formed mostly with s-orbitals with many very weakly bound electrons. But this doesn't helps the superconductivity at all, because the surface of sodium lacks any attractive forces. So that the sodium is soft greasy metal and it doesn't exhibit the superconductivity even at absolute zero temperature. In addition the free gaps between sodium atoms are so large, that when we compress the metallic sodium, the free electrons can hide easily inside of these spaces and they're protected against further compression there.

It's well known, that the superconductors form two main types - the type I works at low temperatures only, whereas the type II superconductors are high-temperature ones. The basic difference between these two types is quantitative only. The choice of atoms with proper combination of attractive and repulsive orbitals is limited with size of periodic table. But when the lattice bindings are involved, then the number of superconductors and degree of electrons squeezing can be increased nearly arbitrarily. What you need is the atom lattice forming large cages, into pores of which the another atoms stripes with excess of movable electrons are embedded. The outer atoms are forming sorta pipes for highly squeezed electron fluid inside of its pores. It's evident, that the higher temperature of superconductivity we want to achieve, the more robust the walls of these pipes must be, i.e, the atoms with orbitals capable of attractive forces must outnumber the atoms with excessive electrons.

What the prof. Eck essentially did was, he selected oxides of carefully chosen mixture of atoms, which differ only slightly with their atom radius. Such an atoms are known to form relatively sparse crystal lattice in similar way, like the atoms within so-called the metallic glasses. Inside of resulting highly aperiodic lattice the binding groups of atoms are sufficiently strong to resist the repulsion of copper atoms, forming the conductive stripes, artificially enriched with electrons with their oxidation. The copper is a metal, which can be oxidized into high but stable oxidation states. The oxidized atoms can attract high number of movable electrons from outside, which do form a dense squeezed clusters around oxidized atoms, like the hungry birds around feeder. The high concentration of movable electrons forms their superconductive state there.

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